Color reproduction is an important factor in the design of color photographic image display materials. Most photographic image display materials, or print materials, are negative-working photographic elements that are exposed by projecting a negative film image onto the print material, with the yellow, magenta, and cyan components of the negative image mediating the blue, green, and red exposure of the print material. Color print materials with silver halide emulsions that are predominantly silver chloride are most useful. Photographic elements in which the emulsions comprise at least 90% silver chloride, and preferably at least 95% silver chloride are most desirable, with less than about 2.0 mole % of iodide, and less than about 5.0 mole % bromide being particularly desirable. Such high chloride emulsions offer several advantages. Perhaps most important to color reproduction is that silver chloride emulsions have essentially no native sensitivity to blue light, unlike silver chlorobromide emulsions that were once commonly used in print materials. For this reason, the blue spectral sensitivity of silver chloride emulsions can be controlled primarily by the choice of sensitizing dyes. Other advantages of silver chloride emulsions include rapid development, ease of bleaching and fixing, and decreased risk of environmental contamination.
Currently commercially available silver chloride color print materials have a near-maximum blue sensitivity to light with a wavelength of about 480 nm. In most cases with chloride emulsions, a single blue sensitizing dye with relatively narrow absorption is used as a sensitizer, so that sensitivity of the emulsion to blue light of much longer or much shorter wavelengths decreases sharply in either direction from the peak sensitivity. Silver bromochloride emulsions with substantial bromide content (for instance, greater than 50% bromide) have a much broader envelope of sensitivity to blue light. Additionally, some silver chloride print materials use more than one sensitizing dye, for instance with one dye that gives a peak sensitivity near 480-485 nm, and another between 475-480 nm.
One reason why most silver chloride emulsions for color paper are sensitized near 480 nm is that such sensitization can help provide adequate blue print speed. Blue print speed, or the exposure time required for exposing the blue-sensitive emulsion, depends on several components, including the spectral distribution of energy from the printer lamp, any lamphouse filtration or other filtration of blue light in the printer, the blue density of the imaging dyes, masking couplers, or other blue density components in the negative being printed, and the spectral distribution of sensitivity in the print material. Many lamps in printers are tungsten sources, that are deficient in blue light relative to red or green light. Tungsten lamps also emit more blue light at 480 nm than at shorter wavelengths. Also, many of the blue density components in the negative have peak absorption of blue light near 440-450 nm. For these reasons, typical silver chloride emulsions with a narrow sensitivity near 480 nm will have faster blue print speed than emulsions with a narrow sensitivity at shorter wavelengths, where the printer lamp emits less energy, and the blue-absorbing components of the negative have a higher density. Emulsions with broader sensitivity will also have higher print speed, including silver chlorobromide emulsions, and silver chloride emulsions with multiple sensitizing dyes with differing peak sensitivities.
Another reason why silver chloride color papers have peak blue sensitivity near 480 nm is the commercial need to maintain compatibility between different brands of color negative originating films, different offerings of these color films from a common manufacturer, and different brands and types of printing equipment and printing materials. Films are formulated such that a neutral exposure scale will eventually result in a neutral print, with neutrality of the scale preserved from low to high density. The contrast attributes and spectral sensitivity of both commercial films and papers affect this. The established commercial product relationships can help explain why blue-sensitive emulsions in AgCl color print papers all have high sensitivity near 480 nm.
It has recently been discovered that spectral sensitization of blue-sensitive emulsions in print materials with predominantly AgCl emulsions, to give a narrow, peak sensitivity of from about 440 to 475 nm, more preferably less than 470 nm, and even more preferably less than 460 nm, causes several desirable effects on the printed tone scale and color reproduction, as discussed in co-pending, commonly assigned U.S. patent application Ser. No. 08/220,989 by Bohan et al., entitled "Improved Photographic Image Display Material and Method of Printing" filed Mar. 31, 1994, the disclosure of which is hereby incorporated by reference in its entirety. In particular, because the peak blue sensitivity in such materials may be separated from the peak green sensitivity by more than about 75 nm, better color separation is seen for colors generated by blue and green exposures. Also, a higher printed blue contrast is observed, giving yellow and green colors, particularly, with higher saturation. A contributing factor to this effect is the better overlap of yellow image dye light absorption in the printed negative with the sensitivity of the hypsochromically sensitized emulsion in the print material. Such predominantly silver chloride emulsions with hypsochromic peak blue sensitivity of from about 440 to 475 nm, and a relatively narrow sensitivity, will hereinafter be referred to as short-blue sensitive emulsions.
The overall color reproduction of a print material is affected by the nature of the dye-forming components as well as the spectral sensitivity of the silver halide emulsions. Commonly, the yellow dye-forming photographic couplers in silver-halide print materials are acylacetanilide compounds. Many such acylacetanilide couplers, when combined with short-blue sensitive emulsions, provide images with higher saturation or print-through contrast, as expected, but also with a visually objectionable increase in the unwanted absorption of green light in yellow and/or green areas of the print. For example, this would make a yellow object appear somewhat orange.
Polymer containing dispersions of yellow photographic couplers have been employed in color print materials, as described in U.S. Pat. No. 4,857,449. Other methods for preparing polymer-containing dispersions of dye-forming couplers are described in U.S. Pat. Nos. 4,939,077; 4,203,716; and 4,840,885. Commonly, these dispersions are prepared from a solution of a coupler, an optional high-boiling solvent, an oil-soluble but water-insoluble polymer, and a volatile organic solvent, which solution is then emulsified and dispersed in an aqueous solution, often comprising water, a hydrophilic colloid such as gelatin, and a surfactant. Other methods describe the formation of loaded latex polymer dispersions using water-miscible or volatile organic solvent. A main advantage of polymer-containing dispersions described in the prior art relates to image preservability to heat and light, although other advantages in manufacturing processes, physical performance of the photographic element, and sensitometric performance have been reported. There has been no previous suggestion, however, to use polymer containing dispersions of yellow dye-forming photographic couplers in combination with short-blue sensitive high-chloride emulsions for improved color reproduction.